How do the channel slopes and velocity effect the headloss formulas in SewerCAD and StormCAD?

Product(s): Bentley SewerCAD, StormCAD
Version(s): V8i, Connect Edition
Area: Output and Reporting

Problem Description

How does the channel slope and velocity effect the headloss formulas in SewerCAD and StormCAD?


In general channel slope is taken into account with velocity and the headloss formula, but is indirectly accounted for. Detailed information on headloss calculations and headloss methods can be found in help documentation, SewerGEMS > File > Help > SewerGEMS Help. 

At a given channel slope, the process outlined below is followed for headloss calculations:

1)     The user enters input data such as diameter and roughness height along with loading/inflows, and the flow is calculated for each pipe.

2)     For a particular pipe, an initial guess is made for the upstream depth and the velocity corresponding to that depth is computed, based on the flow area (V = Q/A)

3)     The Reynolds number is calculated based on that Velocity

4)     The Swamee-Jain equations then used to solve for the friction factor, f

5)     The Darcy-Weisbach equation is then used to solve for the headloss

6)     If the sum of the loss and downstream depth is not comparable to the upstream depth used in step 2, then steps 2-5 are repeated with a better guess of the depth.

7)     The process ends when the depths are within the Hydraulic Grade Convergence Test value in the calculation options.

 At the end of this process you get a headloss value, which is the drop in the HGL from the upstream end to the downstream end. If you take that drop at a given pipe and change its slope by making it much steeper, then you essentially have changed the amount of headloss needed to achieve roughly the same depth. The key is the headloss is the difference between the HGLs and not depths. Therefore by making the pipe steeper the headloss will end up being greater, so the program will use the same iterative approach and ends up with a depth and velocity corresponding to a higher headloss value. So, the slope is indirectly involved as a function of the change in head drop.